The present invention relates to an electronic device for measuring the blood pressure of a patient and a method of operation thereof, and more particularly relates to such a device for blood pressure measurement and such a method of operation thereof of the oscillation method type, in which the accuracy of measurement is maximized while the construction and implementation of the device are kept simple.
Generally, in the oscillation method type of blood pressure measurement device, conventionally a cuff is fitted over the arm of a patient and is inflated by a pressure pump to a pressure somewhat higher than the systolic blood pressure so as to obstruct the blood flow through the patient's arm, thereafter being deflated progressively. During the deflation process, the pressure inside the cuff is measured and a pulse wave signal is obtained therefrom.
In detail, the principles of the conventional or prior art method of measuring blood pressure with such a blood pressure measurement device will be now explained with reference to FIGS. 6 through 8 of the accompanying drawings. FIG. 6 is a graph in which time is shown along the horizontal axis and the pressure of the air (or other fluid) in the cuff is shown along the vertical axis, showing the variation of the cuff pressure during such an episode of inflation and then deflation of the cuff.
First of all, in the same manner as with the conventional stethoscopic method for blood pressure measurement, the cuff is fitted over the patient's arm and is then inflated with a pressure pump to a pressure somewhat higher than the systolic blood pressure (point U on the FIG. 6 graph) so as to obstruct the blood flow through the arteries in said patient's arm. Next, as the pressure in the cuff is steadily and gradually reduced by draining of the fluid from said cuff, after a certain interval a pulsation, shown in FIG. 6 as W, starts to show up in said cuff pressure, this is as a result of the pressure generated by the patient's heart and transmitted to the blood flowing in the arteries of his or her arm, said pressure being transmitted to the cuff and to the fluid therein. The frequency of this pulsation W corresponds to the frequency of the heart beat of the patient. As the cuff pressure further drops, at a certain point this cuff pressure pulsation W disappears, because the pressure on the arteries is diminished and accordingly the blood can flow more smoothly through them. Once the cuff pressure has dropped below the diastolic blood pressure of the patient, no further data can be obtained by the measurement process, and hence at the point V in FIG. 6 the cuff pressure is rapidly reduced, so as to drain the cuff completely.
FIG. 7 shows only the component of the pulse wave form W described above, again with time being shown along the horizontal axis. Hereinafter, this component will be referred to as the pulse wave. And FIG. 8 shows a graph of the amplitude of this pulse wave, against time, with time being shown along the horizontal axis and the pulse wave amplitude being shown along the vertical axis.
Now, the stages of changing of this pulse wave amplitude will be described, with particular reference to FIG. 8. First, from its original substantial zero at the point A, the pulse wave amplitude increases substantially non linearly with time, i.e. with decline of cuff pressure, to a point denoted as S. Thus, the portion of this graph between the points A and S is generally concave upwards with a positive second differential coefficient. From the point S, at which substantially a sharp corner is present, so that the increase rate of the pulse wave amplitude here undergoes a relatively sharp increase, the pulse wave amplitude increases more sharply than before and now substantially linearly with time, up to a point B. From the point B, the pulse wave amplitude increases substantially non linearly with time, i.e. with decline of cuff pressure, through a maximum point denoted as M, to a point D, with the portion of this graph between the points B and D being generally concave downwards with a negative second differential coefficient. Near the point D, the pulse wave amplitude is decreasing relatively quickly with time; but at the point D substantially another sharp corner is present, so that the decrease rate of the pulse wave amplitude here undergoes a relatively sharp decrease, and thereafter the pulse wave amplitude decreases less sharply than before, down to substantially zero at the point C. And it is per se known that cuff pressure at the point S corresponds to the systolic blood pressure, while the cuff pressure at the point D corresponds to the diastolic blood pressure. The oscillation method of blood pressure measurement is based upon these principles, and has the advantage of being relatively immune to external noises, since no microphone is used, and since the frequency of the pulse wave is relatively low, being from about 0.1 Hz to about 10 Hz. In order to implement this oscillation method, there are various per se known electronic blood pressure measuring devices, such a device typically comprising: a cuff for being fitted around the arm of a patient; a means for selectively pressurizing said cuff with fluid, so as to squeeze said arm of said patient; a means for selectively draining said fluid from said cuff either at a relatively rapid rate or at a relatively slow rate; a means for sensing the pressure of said fluid in said cuff and for producing an output signal representative thereof; a means for receiving said pressure signal from said cuff pressure sensing means and for generating therefrom a signal representative of the pulse wave of the patient; and a micro processor or micro computer equipped with a determining means for determining blood pressure values according to the output signals of the pressure sensing means and of the pulse wave detecting means.
However, this type of electronic blood pressure measurement device, and the method of operation thereof, have not yet provided perfect operation. In detail, according to such an electronic blood pressure measurement device and method, the point S in the pulse wave amplitude curve is found by identifying the point at which the increase rate of the pulse wave amplitude undergoes a relatively sharp increase, i.e. the second differential coefficient of the pulse wave amplitude is substantially discontinuous, and the cuff pressure corresponding to this point S is determined as being the systolic blood pressure; but, in an actual pulse wave amplitude curve, the point S at which the pulse wave amplitude thus begins a sharp rise may not be so easy to establish precisely, because it is not always so completely clear and well defined as may appear from the foregoing discussion. Accordingly, the reliability of the measured blood pressure may not be sufficiently high. Further, the sharp of the blood pressure wave form of the patient, and the corresponding shape of the pulse wave amplitude curve, cannot be accurately predicted in advance, because of variation of the measurement conditions and because of individual differences between patients which can be very substantial. The problem thus arises that, if a micro computer program is to be implemented which can accurately determine blood pressure values in spite of such large variations in individual cases in the shape of the pulse wave amplitude curve, the program tends to be so large and voluminous and intricate that the development and debugging of such massive program becomes very difficult. Furthermore, the memory size of the micro computer may not be adequate to the task of storage of the program and/or all the data required therefor, and problems may also arise with regard to the speed of operation of the microcomputer, and sharp increase may occur in the processing time.